Physica D 237 (2008) 2190–2194 www.elsevier.com/locate/physd Passive scalar statistics in a turbulent channel with local time-periodic blowing/suction at walls Guillermo Araya a,∗ , Stefano Leonardi b , Luciano Castillo a a Department of Mechanical, Aeronautical and Nuclear Eng., Rensselaer Polytechnic Institute, Troy, NY, 12180, USA b Department of Mechanical Eng., University of Puerto Rico, Mayaguez, PR, 00680, USA Available online 26 April 2008 Abstract Direct Numerical Simulations (DNS) of an incompressible turbulent channel flow with local forcing at the walls are performed. Time-periodic blowing/suction is applied by means of narrow spanwise slots located at the lower and upper walls in x / L x = 0 (where L x is the channel length). The normal perturbing velocity is varied sinusoidailly in time at several perturbing frequencies between 0.16 < f < 1.6 and at a fixed amplitude of A o = 0.2. The temperature field is also computed and assumed to be a passive scalar. The Reynolds number of the unperturbed case is Re τ = 394 and the Prandtl number is Pr = 0.71. It is concluded that the forcing frequency of f = 0.64 or f + = 0.044 produces the largest local increase of the skin friction in the region 0.1 < x / L x < 0.3, followed by the highest augmentation of the Stanton number. Furthermore, budgets of the passive-scalar variance and wall-normal turbulent heat fluxes at this frequency demonstrate a significant enhancement of the molecular diffusion at the wall and pressure-related terms, respectively. The latter confirms the importance of pressure fluctuations on the transport of passive scalars and redistribution of energy. c  2008 Elsevier B.V. All rights reserved. PACS: 47.27.De; 47.27.E-; 47.27.ek; 47.27.nd Keywords: DNS; Local forcing; Passive scalar 1. Introduction One of the most important characteristics of the near- wall region in turbulent channel flows is the presence of coherent structures [1]. These structures play a key role in the turbulence production, dissipation and transport phenomena in wall-bounded flows. In fact, there have been many attempts to control near-wall turbulence by managing such coherent structures [2]. Among all the techniques employed so far, local forcing is a simple and efficient active approach, which consists of perturbing the flow by a steady or time-periodic velocity (i.e., blowing and/or suction) applied in a confined zone of the wall. Park et al. [3,4] performed experiments in a wind tunnel to analyze the flow structures behind the point, at which a local time-periodic blowing/suction perturbation is applied on a flat plate, by considering integer multiples of the bursting ∗ Corresponding author. E-mail address: araya@mailaps.org (G. Araya). frequency found by Tardu [5]. They showed that, by increasing the forcing frequency, a local reduction in the skin friction, up to 75%, was obtained and significant changes in the downstream structures were observed. Additionally, time-periodic blowing from a spanwise slot was numerically (DNS) investigated by Kim and Sung [6] in an evolving boundary layer at three different forcing frequencies. They obtained maximal increase of the skin friction and streamwise vorticity fluctuations at an optimal blowing frequency of f + = 0.035 downstream of the maximum drag reduction location. Furthermore, the budget analysis of the Reynolds stresses indicated that the greatest augmentation of the pressure–strain term occurred at this frequency. Investigations of the influence of local forcing on turbulent heat transfer are rather scarce. Rhee and Sung [7] numerically predicted the enhancement of heat transfer in a locally forced turbulent, separated and reattaching flow over a backward-facing step. Several forcing frequencies were employed in simulations at a fixed amplitude, namely, 3% of the streamwise time-mean centerline velocity at the inlet, U ∞ . 0167-2789/$ - see front matter c  2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physd.2008.04.011